Stochastic resonance is a phenomenon in both physical and biological systems where a particular level of random noise improves signal detection. In systems where detection is near threshold, a random noise input can improve sensitivity to small amplitude signals by boosting the receptors above threshold. One biological system that could potentially benefit from stochastic resonance is the proprioceptive system, which provides us with the sense of body position and movement. Stochastic resonance could enhance proprioception when movements are novel or must be made precisely. The experiments proposed in this grant application will investigate the potential role of stochastic resonance in the human proprioceptive system at the level of sensory receptors and the level of perception. One of the least understood aspects of proprioception and motor control is the efferent activation of the muscle spindle receptor, an important source of sensory information for coordinating movement. Unlike typical sensory receptors, muscle spindles have both an efferent ("fusimotor") and an afferent nerve supply. The central hypothesis of the proposed project is that stochastic resonance is the mechanism with which the fusimotor system activates muscle spindles. During fusimotor activation, as with the performance of novel or precise movements, the ensuing contraction of intrafusal muscle might provide a low amplitude, random noise stimulus to muscle spindle receptors that are near threshold. A stochastic resonance mechanism could boost muscle spindles above threshold more frequently, thereby improving signal detection. The demonstration of stochastic resonance in muscle spindle afferents is significant for both theoretical and practical reasons. On a theoretical level, stochastic resonance could provide an explanation for an unresolved problem of motor coordination. That is, how the fusimotor system acts to enhance the signal detection properties of the proprioceptive system. On a practical level, the demonstration of stochastic resonance in the human proprioceptive system could lead to improved design of manipulators requiring precise movements, such as in aircraft controls, and sensory enhancement devices for individuals with sensory loss due to diabetic neuropathy or normal aging.